In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming media. Such ink may be phase change ink, aqueous, oil, solvent-based, or UV curable ink or an ink emulsion. A phase change inkjet printer employs phase change inks that are in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. A printhead may then eject the molten ink directly onto an image receiving substrate, or indirectly onto an intermediate imaging member before the imaging member transfers the image to an image receiving substrate. Once the ejected ink is on the image receiving substrate, the ink droplets quickly solidify to form an image. One common image receiving substrate is paper that has been cut into sheets or formed in a continuous media web.
In both the direct and offset printing architecture, images may be formed on a media sheet or a media web. In a web printer, a continuous supply of media, typically provided in a media roll, is mounted onto rollers that are driven by motors. A loose end of the media web is passed through a print zone opposite the printhead or printheads of the printer. Beyond the print zone, the media web is gripped and pulled by mechanical structures so a portion of the media web continuously moves through the print zone. Tension bars or rollers may be placed in the feed path of the moving web to remove slack from the web so the web remains taut without breaking.
In a typical phase change ink direct printing system, melted phase change ink is ejected from inkjets in the printhead directly onto the final receiving web or sheet. In phase change ink continuous-web printing, a high pressure roller nip, also referred to as a spreader, is used after the melted phase change ink is jetted onto the web to spread the ink on the web to achieve the desired print quality. The function of the spreader is to take what are essentially isolated droplets of ink on web and smear them out to make a continuous layer by pressure and/or heat so that spaces between adjacent drops are filled and image solids become more uniform. Other methods of spreading or fixing ink are also possible such as with heat or pressure alone.
In a typical phase change ink indirect printing system, ink is ejected from jets in the printhead onto an intermediate image receiving member, such as a print drum or endless belt. Images formed on the image receiving member are then transferred to a media sheet by passing the media sheet through a nip formed between the image receiving member and a high-pressure transfix roller in a process also referred to as “transfixing” the image.
One difficulty faced in imaging devices, and in particular, imaging devices that utilize phase change ink to form images, is ink abrasion during handling of the prints. Two types of ink abrasion include ink rub and ink offset. Ink rub refers to the smearing or scuffing of the ink of an image on a receiving substrate, such as a sheet of paper. Ink offset refers to ink from an image formed on a surface or portion of a surface of a receiving substrate being transferred to another surface or another portion of the substrate. Ink rub and ink offset are particularly concerns for applications that require extensive handling, such as the outside of envelopes or printed sheets inserted into envelopes. The prior art media sheet depicted in FIG. 7A provides a cross-sectional view of an ink image that includes ink layer 704 on media sheet 708. Ink layer 704 is exposed on the surface of media sheet 708, and is susceptible to ink abrasion.
To prevent ink abrasion, some previously known systems utilize a protective coating, such as varnish, applied over the printed image on the substrate to prevent or minimize damage due to abrasion of the printed image. For example, a varnisher places a protective coating over the entire image to prevent abrasion of ink in the resulting prints. In some previously known systems, overlaying clear ink may require coverages greater than 50% because the act of spreading the ink in the spreader brings all of the ink to a common level and if the overlay of clear ink does not completely cover the colored ink, some of the colored ink may end up at the surface where it is susceptible to abrasion. An example of overlaying clear ink with 100% coverage of an imaged portion of a media sheet is depicted in FIG. 7B. A clear ink layer 712 completely covers ink layer 704 on print medium 708. While high coverage clear coatings applied over images formed on a substrate may be useful to prevent or minimize damage due to ink abrasion of the images, the addition of the clear coating material adds to the expense of the print, and the increased expense due to the coating is commensurate with the amount of coating used per print.
The overall quality of imaged print media also includes a measurement of the glossiness of a print medium after imaging. The glossiness, also referred to as a gloss level, of a printed medium refers to ability of the print medium to reflect light in a specular, or mirror-like, manner with an angle of incident light being approximately equal to the angle of reflected light for a surface with a high gloss level. The factors that affect gloss are the refractive index of the material, the angle of incident light, and the surface topography. A common example of a high-gloss image is a photograph printed on photographic paper, while a common example of a low-gloss or “matte” image is black and white text printed on plain-paper. The ink used to form an image on a print medium may have a different gloss level than the underlying print medium. Certain phase-change inks have gloss levels that exceed the gloss levels of standard printing paper stock. In situations where a high gloss level is desirable, the higher gloss levels of the phase change ink are advantageous. However, the inconsistency between non-imaged portions of the underlying print medium with a low gloss level and imaged portions of the print medium with a high gloss level may detract from the overall appearance of the imaged print medium. Given these challenges, a printing system that protects ink in a print image from damage due to abrasion while producing printed media having uniform gloss levels benefits the field of inkjet printing.